This invention relates to the preparation of polyetherimides.
Polyetherimides are a known class of high performance polymers characterized in particular by resistance to high temperature conditions. At least two methods for their preparation from monomeric reagents are known. The first method includes the steps of displacement of a chloro- or nitro-substituted phthalimide with a salt of a dihydroxyaromatic compound followed by conversion of the resulting bisimide to the corresponding dianhydride and its reaction with at least one diamine. This method, hereinafter designated "displacement-condensation", is disclosed in many publications and patents.
The second method, hereinafter designated "direct displacement", is frequently advantageous by reason of minimization of processing steps and by-product formation. It typically comprises the reaction of a salt of a dihydroxyaromatic compound with a bis(halo- or nitrophthalimide). The direct displacement method is also disclosed in numerous patents. Particular reference is made to U.S. Pat. No. 5,229,482, which describes direct displacement in a solvent of low polarity such as a dichloro- or trichloroaromatic compound in the presence of a phase transfer catalyst which is stable at the relatively high reaction temperatures employed. The phase transfer catalysts having this property which are often preferred are the hexaalkylguanidinium halides, especially chlorides and bromides. A similar direct displacement method, employing an alkoxyaromatic compound such as anisole as a solvent is disclosed in copending, commonly owned application Ser. No. 08/799,886.
It is known that formation of many linear polymers, especially condensation polymers such as polycarbonates, polyesters and polyetherimides, is accompanied by the formation of various proportions of macrocyclic oligomers. These oligomers typically comprise principally molecular species having degrees of polymerization from 2 to about 12 and having an overall cyclic configuration, apart from the presence of any aromatic or heterocyclic rings present in the polymer chain, in contrast to the linear polymers which constitute most of the product.
It is also known that macrocyclic polyetherimide oligomers, hereinafter sometimes simply designated "oligomers" for brevity, can be prepared in high yield by various methods. For example, U.S. Pat. No. 5,357,029 describes their preparation by the reaction of diamines with tetracarboxylic acids or their dianhydrides, and U.S. Pat. No. 5,514,813 by the reaction of salts of dihydroxyaromatic compounds with bis(halo- or nitrophthalimides) in the presence of phase transfer catalysts and in specific molar ratios of reactants. The polymerization of these oligomers is preferably conducted in the presence of a primary amine as initiator and a macrocyclic polyimide polymerization catalyst which may be a metal or hydroxide or salt thereof, as further disclosed in said U.S. Pat. No. 5,357,029.
While macrocyclic oligomers are detected in essentially all polyetherimides, those polyetherimides prepared by displacement-condensation generally contain very low proportions thereof, most often no more than 1% by weight. In such proportions, the macrocyclic oligomers have little or no detrimental effect on the properties of the linear polymer.
It has been discovered, however, that the direct displacement method for preparing linear polyetherimides affords a product having a substantially higher proportion of oligomers. Proportions as high as 10% and most often in the range of about 7-8% by weight are typical.
It is possible to separate oligomers from the linear polyetherimide by such methods as anti-solvent precipitation of linears, whereupon the soluble fraction contains a major proportion of oligomers. The previously known methods for polymerizing said oligomers, however, are disadvantageous in this context since the conditions required therefor are different from those of displacement-condensation and direct displacement.